In the past, air atomization type paint spray guns typically operated with high pressure air to atomize the paint and to adjust the spray pattern between a round pattern and an oval or fan shaped pattern. High pressure air was readily available from compressors and from existing factory air lines and was effective at atomizing a wide range of coating materials. However, the high air pressure tends to produce a less than optimum coating transfer efficiency. Consequently, an undesirable amount of coating material may be dispersed into the atmosphere. Recently, there has been an increased use of high volume low pressure (HVLP) air operated paint spray guns because of the higher transfer efficiency and the resulting decrease in air pollution. In some states such as California, HVLP spray guns operated at 10 psig or less air pressure at the nozzle are exempt from requirements for proving that they meet a minimum transfer efficiency.
HVLP paint spray guns are designed to operate either from a low pressure air source or from a high pressure air source. Typically, a low pressure air source may have an air pressure between 5 and 10 psig while a high pressure air source may have an air pressure between 60 and 100 psig. Guns operated from a low pressure air source have certain disadvantages over guns operated from a high pressure air source. In most cases, high pressure air is already available from an existing air compressor or from an existing high pressure air line in a shop or factory. When a gun is operated from a low pressure source, a separate low pressure turbine must be purchased to operate the spray gun. Such turbines are expensive. Further, a relatively large diameter hose is required to carry the high air flow volumes required to operate the spray gun at a low air pressure. Such hoses are substantially more cumbersome than the smaller diameter high pressure air hoses and consequently increase operator fatigue.
When an HVLP spray gun is operated from a high pressure air source, the high pressure air is metered through either a valve or a fixed orifice to obtain a desired low pressure. When the low pressure supplies both atomization air and fan air, there has been difficulty in accurately controlling the atomization air pressure, especially when the fan air is adjusted. It is critical that the maximum atomization air pressure never exceed 10 psig to meet statutory and regulatory requirements in some jurisdictions. At the same time, it is desirable to have the atomization air pressure close to the maximum permitted 10 psig for improved atomization. When the air pressure is dropped through an orifice or a valve from a high pressure to a low pressure, the pressure of the low pressure air is dependent on air flow. If the low pressure air also supplies fan air orifices, the atomization air pressure will increase when the fan air flow is decreased. If a fixed orifice is sized to give exactly 10 psig, when fan air is totally interrupted, the atomization air pressure may drop to about 5 or 6 psig, for example, with maximum fan air flow. The lower atomization air pressure will adversely affect the paint atomization quality.
Various methods have been used to limit fluctuations in atomization air pressure when fan air flow is changed. In one HVLP spray gun, fan air is controlled by a needle valve. The valve needle has two valve portions forming two valves which operate together, a first of which controls both atomization air and fan air and a second of which controls only fan air. The first valve forms the pressure reducing orifice for dropping the high pressure source air to a desired low pressure. When the valve needle is moved to adjust fan air flow through the second valve, there is a simultaneous adjustment of total air flow through the first valve to limit the atomization air to a predetermined maximum pressure.
U.S. Pat. No. 3,687,368 relates to an electrogasdynamically powered electrostatic spray gun in which the constant flow of atomization air is used to generate an electrostatic voltage. A single air source supplies both the atomization air and fan air. A special bleeder valve is used to prevent changes in the atomization air pressure when fan air is adjusted. As the flow of fan air is decreased, an increased amount of air is vented to the atmosphere to maintain a constant air flow through the gun and hence to maintain a constant atomization air pressure.